crypto: arm/sha256-neon - Fix clang function cast warnings

Instead of casting the function which upsets clang for some reason,
change the assembly function siganture instead.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202304081828.zjGcFUyE-lkp@intel.com/


Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>